(1) Field of the Invention
This invention relates to the generation of audible alarm signals and particularly to the provision of an audible warning which, under the control of an operator, may comprise an amplified voice transmission or a tone which may be varied in frequency and/or repetition rate. More specifically, this invention is directed to communications apparatus which may function as a controllable electronic siren having several operating modes, a public address system or a means for retransmitting the information modulated on a radio frequency transmission. Accordingly, the general objects of the present invention are to provide novel and improved methods and apparatus of such character.
(2) Description of the Prior Art
Audio frequency communication systems capable of multimode operation are well known in the art. A typical utilization of such systems is on emergency vehicles wherein the capability of operating in a siren or public address mode is desired. It is also often desired that such systems, when in the siren mode, produce audible sounds having differing characteristics. Thus, the ability to change from the typical siren "wail" to a "yelp" mode when approaching an intersection is a common requirement. It is also often considered desirable to permit manual control over the tone generated by the siren and/or to provide an operating mode which simulates an air horn. A further feature often required is that the apparatus be capable of operating in a radio repeat mode. In a radio repeat mode calls transmitted by a dispatcher to the emergency vehicle will be amplified and "broadcast" by the siren speaker so that such calls may be heard by personnel who have left the vehicle.
While apparatus has previously been available to satisfy various of the above-discussed requirements, such apparatus has possessed certain deficiencies and limitations. A particularly significant problem has been precipitated by the energy cost dictated switch to smaller size vehicles which has imposed severe space limitations on the installation of electronic equipment. This problem cannot be solved merely by resort to miniaturization and, in fact, attempts to miniaturize equipment through resort to advances in solid state technology have produced equipment failure problems because of the inability to protect circuit components from overheating. Considering the significant power output levels required, potential component damage may result from a short-circuited load, unbalanced input to the power amplifier, operation at a very low frequency in the audio range and other problems.
A further problem in the prior art, which has become of significantly greater interest with increasing fuel costs, is power consumption. Suffice it to say that there is an intense desire in the art to minimize the electrical power consumed by communications apparatus thereby reducing vehicle engine loading and thus fuel consumption.
Still another problem, which has not been effectively solved in the prior art, is the provision of a remote control switch which automatically changes the operational mode of the apparatus. Again by way of example only, the driver of an emergency vehicle with the siren operating in the "wail" mode may wish to change to the "yelp" mode when approaching an intersection. It is desirable that this change be effected without having to divert attention from the road to a control panel. Most desirably, the change in output of the siren should be commanded by either a foot switch or a switch operated by the same control which operates the vehicle horn. However, because some vehicles have electrical systems with the positive terminal grounded whereas other vehicles have a negative ground system, the communications apparatus supplier has previously had to have two different type systems available and there have been the constant problems of equipment damage due to errors in ordering the appropriate apparatus for the particular vehicle.
An additional problem with prior art electronic sirens has resided in the utilization therein of comparatively complex circuitry, including mechanical switches, for changing operating mode and for switching capacitors into and out of the circuit in order to change the frequency of the tone generated by the device.